added ex1.20~1.22

data/book.txt

@@ -0,0 +1,5 @@
+0-201-78345-X 3 20
+0-201-78345-X 2 25
+0-201-78346-X 1 100
+0-201-78346-X 2 99.9
+0-201-78346-X 6 89.9

docs/ex1.20_1.22.md

@@ -0,0 +1,26 @@
+##Exercise 1.20
+
+> http://www.informit.com/title/032174113 contains a copy of Sales_item.h in the Chapter 1 code directory. Copy that file to your working directory. Use it to write a program that reads a set of book sales transactions, writing each transaction to the standard output.
+
+How to test it? use the `book.txt` in `data` folder. And do it like this:
+
+![run](https://db.tt/fm8iHtkF)
+
+[Here](https://github.com/pezy/Cpp-Primer/blob/master/ex1_20.cpp) is the code.
+
+##Exercise 1.21
+> Write a program that reads two Sales_item objects that have the same ISBN and produces their sum.
+
+I think you should confirm the ISBN of two objects. The sum is meaningful when they have the same ISBN.
+
+[Code](https://github.com/pezy/Cpp-Primer/blob/master/ex1_21.cpp)
+
+##Exercise 1.22
+
+> Write a program that reads several transactions for the same ISBN. Write the sum of all the transactions that were read.
+
+Tips: review the example code in "1.4.4. The if Statement".
+
+[Here](https://github.com/pezy/Cpp-Primer/blob/master/ex1_22.cpp) is my code.
+
+![run](https://db.tt/UlkuvpAS)

docs/index.md

@@ -11,3 +11,4 @@ C++ Primer 5th Answer Note
 - [Exercise 1.12 ~ 1.15](/Cpp-Primer/ex1.12_1.15)
 - [Exercise 1.16](/Cpp-Primer/ex1.16)
 - [Exercise 1.17 ~ 1.19](/Cpp-Primer/ex1.17_1.19)
+- [Exercise 1.20](/Cpp-Primer/ex1.20_1.22)

ex1_20.cpp

@@ -0,0 +1,11 @@
+#include <iostream>
+#include "include/Sales_item.h"
+
+int main()
+{
+  Sales_item item;
+  while (std::cin >> item)
+    std::cout << item << std::endl;
+
+  return 0;
+}

ex1_21.cpp

@@ -0,0 +1,14 @@
+#include <iostream>
+#include "include/Sales_item.h"
+
+int main()
+{
+  Sales_item item1, item2;
+  std::cin >> item1 >> item2;
+  if (item1.isbn() == item2.isbn())
+    std::cout << item1 + item2 << std::endl;
+  else
+    std::cout << "They have the different ISBN." << std::endl;
+
+  return 0;
+}

ex1_22.cpp

@@ -0,0 +1,22 @@
+#include <iostream>
+#include "include/Sales_item.h"
+
+int main()
+{
+  Sales_item item, sum;
+  while (std::cin >> item)
+  {
+    if (item.isbn() != sum.isbn())
+    {
+      if (!sum.isbn().empty())
+        std::cout << sum << std::endl;
+      sum = item;
+    }
+    else
+      sum += item;
+  }
+
+ std::cout << sum << std::endl;
+
+  return 0;
+}

include/Sales_item.h

@@ -0,0 +1,142 @@
+/*
+ * This file contains code from "C++ Primer, Fifth Edition", by Stanley B.
+ * Lippman, Josee Lajoie, and Barbara E. Moo, and is covered under the
+ * copyright and warranty notices given in that book:
+ * 
+ * "Copyright (c) 2013 by Objectwrite, Inc., Josee Lajoie, and Barbara E. Moo."
+ * 
+ * 
+ * "The authors and publisher have taken care in the preparation of this book,
+ * but make no expressed or implied warranty of any kind and assume no
+ * responsibility for errors or omissions. No liability is assumed for
+ * incidental or consequential damages in connection with or arising out of the
+ * use of the information or programs contained herein."
+ * 
+ * Permission is granted for this code to be used for educational purposes in
+ * association with the book, given proper citation if and when posted or
+ * reproduced.Any commercial use of this code requires the explicit written
+ * permission of the publisher, Addison-Wesley Professional, a division of
+ * Pearson Education, Inc. Send your request for permission, stating clearly
+ * what code you would like to use, and in what specific way, to the following
+ * address: 
+ * 
+ *     Pearson Education, Inc.
+ *     Rights and Permissions Department
+ *     One Lake Street
+ *     Upper Saddle River, NJ  07458
+ *     Fax: (201) 236-3290
+*/ 
+
+/* This file defines the Sales_item class used in chapter 1.
+ * The code used in this file will be explained in
+ * Chapter 7 (Classes) and Chapter 14 (Overloaded Operators)
+ * Readers shouldn't try to understand the code in this file
+ * until they have read those chapters.
+*/
+
+#ifndef SALESITEM_H
+// we're here only if SALESITEM_H has not yet been defined 
+#define SALESITEM_H
+
+// Definition of Sales_item class and related functions goes here
+#include <iostream>
+#include <string>
+
+class Sales_item {
+// these declarations are explained section 7.2.1, p. 270 
+// and in chapter 14, pages 557, 558, 561
+friend std::istream& operator>>(std::istream&, Sales_item&);
+friend std::ostream& operator<<(std::ostream&, const Sales_item&);
+friend bool operator<(const Sales_item&, const Sales_item&);
+friend bool 
+operator==(const Sales_item&, const Sales_item&);
+public:
+    // constructors are explained in section 7.1.4, pages 262 - 265
+    // default constructor needed to initialize members of built-in type
+    Sales_item() = default;
+    Sales_item(const std::string &book): bookNo(book) { }
+    Sales_item(std::istream &is) { is >> *this; }
+public:
+    // operations on Sales_item objects
+    // member binary operator: left-hand operand bound to implicit this pointer
+    Sales_item& operator+=(const Sales_item&);
+
+    // operations on Sales_item objects
+    std::string isbn() const { return bookNo; }
+    double avg_price() const;
+// private members as before
+private:
+    std::string bookNo;      // implicitly initialized to the empty string
+    unsigned units_sold = 0; // explicitly initialized
+    double revenue = 0.0;
+};
+
+// used in chapter 10
+inline
+bool compareIsbn(const Sales_item &lhs, const Sales_item &rhs) 
+{ return lhs.isbn() == rhs.isbn(); }
+
+// nonmember binary operator: must declare a parameter for each operand
+Sales_item operator+(const Sales_item&, const Sales_item&);
+
+inline bool 
+operator==(const Sales_item &lhs, const Sales_item &rhs)
+{
+    // must be made a friend of Sales_item
+    return lhs.units_sold == rhs.units_sold &&
+           lhs.revenue == rhs.revenue &&
+           lhs.isbn() == rhs.isbn();
+}
+
+inline bool 
+operator!=(const Sales_item &lhs, const Sales_item &rhs)
+{
+    return !(lhs == rhs); // != defined in terms of operator==
+}
+
+// assumes that both objects refer to the same ISBN
+Sales_item& Sales_item::operator+=(const Sales_item& rhs) 
+{
+    units_sold += rhs.units_sold; 
+    revenue += rhs.revenue; 
+    return *this;
+}
+
+// assumes that both objects refer to the same ISBN
+Sales_item 
+operator+(const Sales_item& lhs, const Sales_item& rhs) 
+{
+    Sales_item ret(lhs);  // copy (|lhs|) into a local object that we'll return
+    ret += rhs;           // add in the contents of (|rhs|) 
+    return ret;           // return (|ret|) by value
+}
+
+std::istream& 
+operator>>(std::istream& in, Sales_item& s)
+{
+    double price;
+    in >> s.bookNo >> s.units_sold >> price;
+    // check that the inputs succeeded
+    if (in)
+        s.revenue = s.units_sold * price;
+    else 
+        s = Sales_item();  // input failed: reset object to default state
+    return in;
+}
+
+std::ostream& 
+operator<<(std::ostream& out, const Sales_item& s)
+{
+    out << s.isbn() << " " << s.units_sold << " "
+        << s.revenue << " " << s.avg_price();
+    return out;
+}
+
+double Sales_item::avg_price() const
+{
+    if (units_sold) 
+        return revenue/units_sold; 
+    else 
+        return 0;
+}
+#endif